This invention relates to implantable medical devices (IMDs) and their various components, including flat electrolytic capacitors for same, and methods of making and using same, particularly to an electrode stack assembly having a plurality of capacitor layers formed of tailored numbers of anode layers of selected capacitor layers to tailor the stack assembly height to fill the available stack height space in the capacitor case.
As described in the above-referenced parent patent application Ser. No. 09/103,843, and the provisional application that it claims priority from, a wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high energy cardioversion and/or defibrillation shocks to a patient""s heart when a malignant tachyarrhythmia is detected. Current ICDs typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier developed automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Generally speaking, it is necessary to employ a DC-DC converter within an ICD implantable pulse generator (IPG) to convert electrical energy from a low voltage, low current, electrochemical cell or battery enclosed within the IPG housing to a high voltage energy level stored in one or more high energy storage capacitor, as shown for example, in commonly assigned U.S. Pat. No. 4,548,209. The conversion is effected upon confirmation of a tachyarrhythmia by a DC-DC xe2x80x9cflybackxe2x80x9d converter which includes a transformer having a primary winding in series with the battery and a secondary winding in series with the high energy capacitor(s) and an interrupting circuit or switch in series with the primary coil and battery that is periodically opened and closed during a charging cycle. Charging of the high energy capacitor is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding when the switch is closed. The field collapses when the current in the primary winding is interrupted by opening the switch, and the collapsing field develops a current in the secondary winding which is applied to the high energy capacitor to charge it. The repeated interruption of the supply current charges the high energy capacitor to a desired level of several hundred volts over a charging time of the charge cycle. Then, the energy is rapidly discharged from the high voltage capacitor(s) through cardioversion/defibrillation electrodes coupled to the IPG through ICD leads and arranged about or in a heart chamber or vessel if the tachyarrhythmia is confirmed as continuing at the end of the charge time. The cardioversion/defibrillation shocks effected by discharge of such capacitors are typically in the range of about 25 to 40 Joules. The process of delivering cardioversion/defibrillation shocks in this way may be repeated if an earlier delivered cardioversion/defibrillation shock does not convert the tachyarrhythmia to a normal heart rhythm.
Energy, volume, thickness and mass are critical features in the design of ICD pulse generators that are coupled to the ICD leads. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrillation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality to the ICD IPG. It is also desirable to provide such ICD IPGs at low cost while retaining the highest level of performance. At the same time, reliability of the capacitors cannot be compromised.
Various types of flat and spiral-wound capacitors are known in the art, some examples of which are described as follows and/or may be found in the patents listed in Table 1 of the above-referenced parent patent application Ser. No. 09/103,843.
Prior art high voltage electrolytic capacitors used in ICDs have two or more anode and cathode layers (or xe2x80x9celectrodesxe2x80x9d) and operate at room or body temperature. Typically, the capacitor is formed with a capacitor case enclosing an etched aluminum foil anode, an aluminum foil or film cathode, and a Kraft paper or fabric gauze spacer or separator impregnated with a solvent based liquid electrolyte interposed therebetween. A layer of aluminum oxide that functions as a dielectric layer is formed on the etched aluminum anode, preferably during passage of electrical current through the anode. The electrolyte comprises an ion producing salt that is dissolved in a solvent and provides ionic electrical conductivity between the cathode and the aluminum oxide dielectric. The energy of the capacitor is stored in the electrostatic field generated by opposing electrical charges separated by the aluminum oxide layer disposed on the surface of the anode and is proportional to the surface area of the aluminum anode. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor""s overall (i.e., external) dimensions. The separator material, anode and cathode layer terminals, internal packaging, electrical interconnections, and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, these and other components in a capacitor and the desired capacitance limit the extent to which its physical dimensions may be reduced.
Some ICD IPGs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrilliators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, December 1989, Year Book Medical Publishers, Chicago, and as described in U.S. Pat. No. 4,254,775. The electrodes or anode and cathodes are wound into anode and cathode layers separated by separator layers of the spiral. Anode layers employed in such photoflash capacitors typically comprise one or two sheets of a high purity (99.99%), porous, highly etched, anodized aluminum foil. Cathode layers in such capacitors are formed of a non-porous, highly etched aluminum foil sheet which may be somewhat less pure (99.7%) respecting aluminum content than the anode layers. The separator formed of one or more sheet or layer of Kraft paper saturated and impregnated with a solvent based liquid electrolyte is located between adjacent anode and cathode layers. The anode foil thickness and cathode foil thickness are on the order of 100 micrometers and 20 micrometers, respectively. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle, highly etched aluminum anode foils from fracturing during winding of the anode, cathode and separator layers into a coiled configuration. The cylindrical shape and paper core of commercial photoflash capacitors limits the volumetric packaging efficiency and thickness of an ICD IPG housing made using same.
The aluminum anodes and cathodes of aluminum electrolytic capacitors generally each have at least one tab extending beyond their perimeters to facilitate electrical connection of all (or sets of) the anode and cathode layers electrically in parallel to form one or more capacitor and to make electrical connections to the exterior of the capacitor case. Tab terminal connections for a wound electrolytic capacitor are described in U.S. Pat. No. 4,663,824 that are laser welded to feedthrough pin terminals of feedthroughs extending through the case. Wound capacitors usually contain two or more tabs joined together by crimping or riveting.
Flat electrolytic capacitors have also been disclosed in the prior art for general applications as well as for use in ICDs. More recently developed ICD IPGs employ one or more flat high voltage capacitor to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. For example, U.S. Pat. No. 5,131,388 discloses a flat capacitor having a plurality of stacked capacitor layers. Each capacitor layer contains one or more anode sheet forming an anode layer having an anode tab, a cathode sheet or layer having a cathode tab and a separator for separating the anode layer from the cathode layer. In the ""388 patent, the electrode stack assembly of stacked capacitor layers is encased within a non-conductive, polymer envelope that is sealed at its seams and fitted into a chamber of a conductive metal, capacitor case or into a compartment of the ICD IPG housing, and electrical connections with the capacitor anode(s) and cathode(s) are made through feedthroughs extending through the case or compartment wall. The tabs of the anode layers and the cathode layers of all of the capacitor layers of the stack are electrically connected in parallel to form a single capacitor or grouped to form a plurality of capacitors. The aluminum anode layer tabs are gathered together and electrically connected to a feedthrough pin of an anode feedthrough extending through the case or compartment wall. The aluminum cathode layer tabs are gathered together and electrically connected to a feedthrough pin of a cathode feedthrough extending through the case or compartment wall or connected to the electrically conductive capacitor case wall.
Many improvements in the design of flat aluminum electrolytic capacitors for use in ICD IPGs have been disclosed, e.g., those improvements described in xe2x80x9cHigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d presented by P. Lunsmann and D. MacFarlane at CARTS 96: 16th Capacitor and Resistor Technology Symposium, Mar. 11-15 1996, and at CARTS-EUROPE 96: 10th European Passive Components Symposium., Oct. 7-11 1996, pp. 35-39. Further features of flat electrolytic capacitors for use in ICD IPGs are disclosed in U.S. Pat. Nos. 4,942,501; 5,086,374; 5,146,391; 5,153,820; 5,562,801; 5,584,890; 5,628,801; and 5,748,439, all issued to MacFarlane et al.
A number of recent patents including U.S. Pat. Nos. 5,660,737 and 5,522,851; 5,801,917; 5,808,857; 5,814,082; 5,908,151; 5,922,215; 5,926,357; 5,930,109; 5,968,210 and 5,983,472, all assigned to the same assignee, disclose related flat electrolytic capacitor designs for use in ICDs. In several of these patents, internal alignment elements are employed as a means for controlling the relative edge spacing of the anode and cathode layers from the conductive capacitor case. In these patents, each anode layer and cathode layer is provided with an outwardly extending tab, and the anode and cathode tabs are electrically connected in common to a feedthrough pin and a step feature of the conductive capacitor case, respectively. The cathode tabs are gathered together against the step feature and ultrasonically welded together and to the step feature. In the ""357 patent, the anode tabs are laser welded to one end of an aluminum ribbon that is ultrasonically welded at its other end to an aluminum layer that is ultrasonically welded to the terminal pin. The feedthrough terminal pin is electrically isolated from and extends outside and away from the case to provide an anode connection pin. A cathode connection pin is attached to the case and extends outwardly therefrom. The anode and cathode connection pins are electrically connected into the DC-DC converter circuitry, but the attachment mechanism is not described in any detail.
As noted above, the capacitor layers of a flat electrode stack assembly typically comprise at least one anode layer, a cathode layer and a separator formed of one or more separator sheet. However, it is known to employ two or more highly etched aluminum foil sheets to form an anode layer of each capacitor layer. The above referenced ""890 patent shows three highly etched anode foils or sheets stacked together, and the above-referenced ""082 patent suggests single, double, triple or higher multiple anode sheets in each capacitor layer. These suggested capacitor layers have the same selected number of anode sheets having the same anode sheet thickness and therefor would be of uniform thickness for any given capacitor stack. Therefore, all of the capacitor layers or anode-cathode sub-assemblies of a electrode stack assembly would be of the same thickness or height.
It is desirable to achieve the maximum surface area and capacitance of the electrode stack assembly and minimize empty height space of the interior case chamber without causing undue pressure on the electrode stack assembly as the separator swells upon electrolyte filling.
Accordingly, the present invention is directed to providing efficient usage of the space within the interior case chamber of an electrolytic capacitor particularly adapted for use in IMDs. The capacitor is formed with a capacitor or electrode stack assembly having a stack assembly thickness or height HN that is tailored to fit a case wall height Hcw of the capacitor case with minimal wasted space and allowance for any stack height tolerance to. The electrode stack assembly comprises a plurality of N stacked capacitor layers each having a specified capacitor layer thickness or height. The N capacitor layers are preferably formed of a cathode layer, and anode sub-assembly and at least one separator layer comprising one or more separator sheet on either side of the cathode layer and the anode sub-assembly.
At least N1 capacitor layers have a first capacitor layer thickness TCL1 and N2 capacitor layers have a second capacitor layer thickness TCL2 where N=N1+N2, and HN=N1*TCL1+N2*TCL2.(plus the thickness of additional upper and lower separator layers, if present) The anode sub-assemblies of the N1 capacitor layers comprising x anode layers each having anode layer thickness tx that are stacked together, each anode sub-assembly having an anode sub-assembly thickness Tx. Similarly, the anode sub-assemblies of the N2 capacitor layers comprising y anode layers each having anode layer thickness ty that are stacked together, each anode sub-assembly having an anode sub-assembly thickness Ty.
In one thickness tailoring embodiment, the x anode layers each have the same anode layer thickness tx, the y anode layers each have the same anode layer thickness ty, tx=ty, and therefore the condition xxe2x89xa0y is necessary in order to achieve differing anode sub-assembly thicknesses Tx and Ty. In a second tailoring embodiment, the x anode layers each have the same anode layer thickness tx, the y anode layers each have the same anode layer thickness ty, but txxe2x89xa0ty, and therefore either condition xxe2x89xa0y or x=y is sufficient in order to achieve differing anode sub-assembly thicknesses Tx and Ty. In a third tailoring embodiment, certain or all of the x anode layers have differing anode layer thicknesses tx1, tx2, et seq., and certain or all of the y anode layers have differing anode layer thicknesses ty1, ty2, et seq., and tx1xe2x89xa0ty1, tx2xe2x89xa0ty2, et seq., and therefore either condition xxe2x89xa0y or x=y is sufficient in order to achieve differing anode sub-assembly thicknesses Tx and Ty. In a fourth tailoring embodiment, certain or all of the x anode layers have differing anode layer thicknesses tx1, tx2, et seq., and certain or all of the y anode layers have differing anode layer thicknesses ty1, ty2, et seq., and tx1=ty1, tx2=ty2, et seq., and therefore the condition xxe2x89xa0y is necessary in order to achieve differing anode sub-assembly thicknesses Tx and Ty.
In a preferred embodiment the electrolytic capacitor is formed of a capacitor case defining an interior case chamber and case chamber periphery, an electrode stack assembly of a plurality of stacked capacitor layers having anode and cathode tabs disposed in the interior case chamber, an electrical connector assembly for providing electrical connection with the anode and cathode tabs through the case, a cover, and electrolyte filling the remaining space within the interior case chamber. A case liner can also be disposed around the electrode stack assembly periphery, and its upper and lower wall thicknesses are taken into account in specifying the stack height tolerance to.
The number N of capacitor layers and the overall electrode stack assembly thickness or stack height HN of the N stacked capacitor layers that are fitted into the interior case chamber depends on the specified case side wall height Hcw, and the stack height tolerance to providing for variances in the stack thickness and any stack binders or liners holding the stacked capacitor layers together and/or isolating the stack periphery from the case side wall. The stack tolerance to is defined to ensure that the electrode stack assembly, with or without a liner, fits into the interior case chamber after assembly and to allow for separator swelling upon filing with electrolyte. The total electrode stack assembly thickness or height HN is dependent upon the total number N of capacitor layers and the thickness T1, T2, . . . Tn of the selected groups N1, N2, . . . Nn of capacitor layers. The capacitor layer thickness T1, T2, . . . Tn depends on the number and the thickness of the anode foils or sheets of the anode layers, the thickness of the cathode layers, and the thickness of the separator sheets, particularly when swollen by liquid electrolyte. By this selection, the maximum surface area and capacitance of the electrode stack assembly is achieved and empty height space of the interior case chamber is minimized without causing undue pressure.
Those of ordinary skill in the art will understand immediately upon referring to the drawings, detailed description of the preferred embodiments and claims hereof that many objects, features and advantages of the capacitors and methods of the present invention will find application in the fields other than the field of IMDs.